It is known to decompose a received signal into in-phase (I) and quadrature (Q) signal components for demodulation purposes. This approach may be used for signals modulated using frequency modulation (FM), quadrature amplitude modulation (QAM), and other such modulation techniques. Ideally, the I and Q components have identical amplitudes, and phases that differ by a particular amount.
In practical implementations, a signal may have I and Q components with dissimilar amplitudes, or the I and Q components may have a phase difference that is not ideal. Such deviations from the ideal phase difference and amplitude are commonly referred to as phase and amplitude imbalance. An imbalance may result from imperfections in circuitry, stray capacitance, signal routing, and the like. The imbalance may be introduced while receiving the modulated signal, or may originate during construction of the signal.
When phase or gain imbalance distorts the received signal, subsequent signal processing is impacted. The prior art has long used higher tolerance components in an attempt to avoid phase and/or amplitude imbalance between the I and Q components. Such an approach has significant cost impact and may still not adequately address the problem. Another prior art approach attempts to account for imbalance by estimating and removing such imbalance. One such approach is described in U.S. Pat. No. 5,396,656 issued on Mar. 7, 1995, to Jasper et al., for a Method For Determining Desired Components Of Quadrature Modulated Signals. Here, a closed loop feedback technique is used to continuously determine an error signal by updating estimates of an imbalance component until the magnitude of the error signal is negligible. Yet another approach is described in U.S. Pat. No. 4,122,448 issued on Oct. 24, 1978, to Martin, for an Automatic Phase And Gain Balance Controller For A Baseband Processor. Martin uses a pilot signal to obtain phase and amplitude imbalances, and these imbalances are corrected using a feedback circuit.
Prior art approaches to solving the problem of phase and amplitude imbalance between in-phase and quadrature components of a modulated signal vary in complexity and effectiveness. It is desired in the art to provide an effective solution with reduced circuitry and/or signal processing requirements. Therefore, a new method and apparatus for adjusting imbalance between in-phase and quadrature components of a received signal is needed.